This invention relates to a hydraulic power booster, in particular for the actuation of a master cylinder in an automotive vehicle brake system, with a booster piston. A brake valve which is essentially arranged in parallel with the booster piston and by means of which hydraulic pressure may be adapted to be metered into a booster chamber dependent on the actuating force applied to the brake valve, the booster chamber being defined by a pedal-operable piston rod and by the booster piston.
A brake device featuring the above characteristics is known from DE-OS (German Laid-open Print) No. 31 08 908.9. In the known brake booster, a master cylinder piston is arranged in a first cylinder bore, forming one component together with a booster piston. A web connects the master cylinder piston and the booster piston with each other. A pedal-side front face of the booster piston confines a booster chamber formed within the housing and is arranged coaxially relative to a pedal-operable piston rod. Hinged to the piston rod is a lever arrangement for the actuation of a brake valve. By exerting a force on the piston rod and by actuating the brake valve, pressure fluid may be metered into the booster chamber proportional to the actuating force to displace the booster piston and the master cylinder piston connected with the booster piston in the actuating direction of the brake booster. Thus, a corresponding pressure will be built up in the working chamber of the master cylinder. Connected to the working chamber of the master cylinder are wheel brakes that are pressurized in this way. A corresponding slowing-down of the vehicle will ensue after any initiation of any such brake actuating force.
Upon pressurization of the booster chamber, the piston rod connected with the brake pedal will likewise be pressurized. As a result, a corresponding reaction force will be felt at the pedal which will provide feed-back information to the driver of the vehicle with regard to the intensity of the braking action initiated by him. Under certain operating conditions, however, it may be desirable to have a certain volume of fluid supplied to the wheel brakes even before a noticeable reaction force is felt at the brake pedal.